The invention relates to a method for treating impurities contained in exhaust gases of ships, the method comprising                scrubbing with water the exhaust gases in an exhaust gas scrubber in order to reduce sulphur dioxide emissions of the exhaust gases,        supplying wash water to be purified, containing impurities and exiting from the exhaust gas scrubber, to a purification unit onboard a ship, in which effluent is purified, and        monitoring a pH value of the purified effluent, and if it is less than 6.5, the pH value of the effluent is adjusted to a value of at least 6.5, after which the purified effluent is discharged into a sea or returned to the exhaust gas scrubber.        
The invention also relates to a ship comprising an exhaust gas scrubber for purifying exhaust gases from the ship's combustion engine and for reducing sulphur dioxide emissions, and a purification unit for purifying wash water to be purified and exiting from the exhaust gas scrubber.
The invention further relates to a purification unit for purifying polluted wash water, i.e. effluent, exiting from an exhaust gas scrubber of a ship.
From US 2010224070 A1 a method of aforesaid kind and a ship of the above type are known.
It is widely known to clean exhaust gases from ships' engines by means of exhaust gas scrubbers. The purpose is to reduce sulphur dioxide emissions of the exhaust gases in particular since they are problematic to the environment. Sulphur dioxide emissions occur because engines of a ship use a sulphur-containing fuel, which oxidizes during fuel combustion process in the engine. Low sulphur fuels enable sulphur dioxide emissions to be reduced but the low sulphur fuels are expensive. In order to be able to use fuels whose sulphur content can be relatively high, i.e. higher than that in so-called low sulphur fuels, exhaust gases are scrubbed in exhaust gas scrubbers, enabling sulphur dioxide emissions to be dramatically reduced as compared with not scrubbing the exhaust gases. An exhaust gas scrubbing process produces wash water which contains impurities and which as such cannot be discharged into the sea since the wash water contains a large amount of impurities and, typically, the pH value of the wash water is also too low for it to be allowed to be discharged into the sea. Wash water exiting from the exhaust gas scrubber may be fed back to the exhaust gas scrubber, but the wash water cannot be recirculated “endlessly” in this way since the wash water only becomes more polluted the more exhaust gases are scrubbed therewith. Prior to being discharged into the sea, wash water has been diluted with water, but this does not eliminate the environmental problems caused by the effluent. No success has been achieved in purifying the wash water well enough to enable it to be discharged from the ship into the sea. A procedure according to which the wash water is collected to large tanks onboard the ship wherefrom it is pumped at a harbour and transported to purification plants ashore is, due to the large amount of wash water, so laborious that it is not implementable in practice.
The above-described ways to treat exhaust gases and impurities transferred from the exhaust gas scrubber to the wash water do not provide a good end result (do not eliminate impurities as efficiently as desired) or are complicated (e.g. the transfer of wash water from the ship to the shore) and necessitate the use of chemicals.
From U.S. Pat. No. 6,810,662 B2 an arrangement is known for purifying exhaust gases from a combustion engine in exhaust gas scrubbers. However, the document does not disclose how to treat wash water or reaction products produced when water reacts with compounds contained in the exhaust gases.
From GB 2288342 an arrangement is known for purifying exhaust gases from a combustion engine in an exhaust gas scrubber. The document discloses purification of wash water in a purification unit in which the solid impurity particles contained in the wash water are collected in a filter. The problem is, however, that the described purification unit is incapable of treating and removing from the wash water impurities dissolved therein, or extremely small impurity particles, either.